There are several health benefits.  It is a probiotic, for one, which helps them develop good gut flora.  It also helps you, because it inhibits algae growth in the waterer.  A quick search on here will turn up lots of threads on its benefits.
The reason for not putting it in galvanized waterers is that it leaches zinc (I think it's zinc, anyway) out of the metal to the point that it can be toxic for them.
I have mostly read to use between 1 tsp and one Tbsp per gallon.  Truth is, I slosh a little in, probably around 1 tsp.
It needs to be the organic version, by the way.  Bragg's is a brand commonly found in grocery stores.  The benefits come from the organic version, which has the mother, which is the beneficial organisms.
It's good for people, too.